Paget's disease (PD) is a chronic bone disease characterized by focal regions of highly exaggerated and disordered bone remodeling. The etiology of PD has been difficult to define, as there are clearly both genetic and environmental contributions to the disease. Recently, disease-associated mutations have been identified in the SQSTM1/p62 gene in cases of both familial and sporadic PD, and mutations of this gene may be linked to up to 30% of all Paget's cases. The SQSTM1/p62 gene encodes a 62 kDa protein that functions in multiple signal transduction pathways involved in normal osteoclastogenesis, including those activated by RANKL, TNF, and IL-1. We hypothesize that p62 mutations lead to increased activation of one or more of these pathways in osteoclast precursors, resulting in increased osteoclastogenesis and susceptibility to the subsequent development of PD. We further hypothesize that expression of mutant p62 in osteoblasts or bone marrow stromal cells enhances their capacity to support osteoclastogenesis as well as increase new bone formation in response to the increased osteoclast activity. To investigate these hypotheses, we have developed a mouse model of Paget's patients with p62 mutations by introducing a P394L mutation (analogous to the human Paget's-associated P392L mutation) into the endogenous mouse p62 gene. We now propose a series of complementary in vivo and in vitro experiments employing this newly developed mouse model to determine the extent to which this mutation contributes to the full pagetic phenotype, and to determine the effect of this mutation on the signaling pathways that regulate both osteoclast and osteoblast formation, function, and survival. This unique mouse model will allow us to determine the specific mechanisms by which p62 mutation contributes to the development of PD, and will also serve as a valuable research tool for identifying other environmental and genetic factors contributing to PD. PUBLIC HEALTH RELEVANCE: PD is the second most common bone disease after osteoporosis, and results in significant pain, deformity, increased susceptibility to fractures, and a variety of neurological complications in Paget's patients. However, despite the prevalence of PD, it is relatively understudied and its origin is poorly understood. The proposed studies should lead to a better understanding of PD, and may therefore facilitate the development of improved therapeutic approaches.